Threaded caps used to seal correspondingly threaded containers are generally known as screw caps. Screw caps for containers are ideally tightened to a predetermined torque. This predetermined torque is selected so that it will close the container sufficiently tightly to avoid loss, deterioration, or contamination of the contents during transportation and storage. The torque may not be too high, however, or the screw cap will be tightened so tightly that it cannot be opened manually, or that it damages the screw cap, the container, or both. This predetermined torque is generally referred to as the application torque.
Acceptable tightness is generally determined by running a number of containers through the capping process, then measuring the torque required to remove the screw cap. The torque necessary to remove properly tightened containers is generally known as the removal torque. The removal torque must then be correlated with the application torque which was used to produce it in order to determine an optimum application torque. This correlation may vary from machine to machine and between the various spindles on a turret.
In addition, this correlation is generally container and cap specific. There are a number of factors specific to a cap and container that influence how much application torque is required to produce the desired removal torque. Current means of determining the optimum application torque are limited because they rely on evaluation of the torque used to apply screw caps to similarly shaped mock containers, rather than to the production container itself. Because the required application torque may vary in response to such seemingly unrelated factors as color of the dye used in the plastic from which the container is formed, the use of mock containers provides an imprecise measurement of the application torque required.
Current means of measuring the application torque are also limited because they are based on a single maximum torque which is reached during the application process. That maximum torque provides limited and potentially misleading information about the capping process and potentially about when the maximum torque was reached during this process. This single maximum torque does not necessarily correlate to the application torque needed to create a particular removal torque. For example, an artificially high maximum torque may be recorded if the screw cap is initially cross threaded, creating a peak torque early in the application process. As the screw cap slips back into proper thread alignment and then tightens properly it may only reach a torque lower than the application torque recorded earlier when the screw cap was cross threaded. There may also be problems setting the application torque if the desired removal torque is achieved by a wide range of application torques, or by a range of application torques which are internally inconsistent. In this case, it would be helpful to have more complete application torque information. If such information is available it can be analyzed to help identify capping problems or which torque patterns are most typical.
Finally, it would be useful to have the ability to independently check the torque applied by the capper machine for accuracy. The capper machine itself may be malfunctioning in a way that makes it appear to be applying a certain torque during the capping process when the application torque is actually different than the capping machine is reporting.
Thus there exists a need for a method and apparatus to measure the amount of torque which is applied when tightening specific cap and production container combinations, when using a specific machine or spindle on a particular turret, or which can generate a plurality of measurements that can be used to evaluate the torque which is applied throughout the capping process.